1. Field of the Invention
The present invention relates to a coordinate input apparatus and its control method. More particularly, the invention relates to a coordinate input apparatus and its control method in which a vibration of an elastic wave input from a vibration input pen is detected with a plurality of sensors mounted on a vibration propagation plate, and in accordance with the propagation time of this elastic wave vibration, the coordinate of a vibration input point is detected.
2. Related Background Art
As disclosed in JP-B-5-62771, a coordinate input apparatus using ultrasonic waves calculates a coordinate value of an input position by detecting a delay time of a wave propagating on a tablet surface as an input plane. A coordinate input apparatus of this type is not necessary to form conductive wires in a matrix shape on a tablet so that the apparatus can be realized with a low cost. If a transparent plate glass is used as a tablet, a coordinate input apparatus having transparency higher than other coordinate detecting methods can be realized.
With the structure of a general coordinate input apparatus using ultrasonic waves, a plurality of vibration sensors for converting mechanical vibrations into electric signals, such as piezoelectric elements, are fixed to a vibration propagation plate, and a time from when vibrations are generated by a vibration input pen to when the vibration is detected with each vibration sensor is measured to calculate the vibration input position therefrom.
A signal detected with a vibration sensor is weak and not suitable for signal processing. Therefore, in general, this weak signal is amplified by a pre-amplifier. The gain of the pre-amplifier is set so that the maximum value of the weak signal amplified becomes lower than a power source voltage of a signal processing circuit.
In such a coordinate input apparatus using elastic waves, the level of a detection signal changes greatly with a vibration propagation distance, a pen pressure, a pen angle, a component variation and the like. A threshold value for signal detection is generally determined by considering a threshold value change amount to be caused by the above-described factors, an electromagnetic noise level, and a power source voltage.
However, if a coordinate input apparatus uses A0 lamb waves, unnecessary S0 wave vibrations are generated. The minimum level (threshold level) for detecting A0 waves is limited by the maximum level of such unnecessary vibrations.
The level of an S0 wave is generally about 3 to 9% of an A0 wave, and is very large as compared with an ordinary electromagnetic noise level. If the detection threshold level is set so that it cannot detect the S0 wave, the effective signal amplitude range of the A0 wave to be detected becomes very narrow. Therefore, the A0 wave may become difficult to exceed the detection level because of the above-described factors (vibration propagating distance, pen pressure, pen angle and the like). Accordingly, the problem of a difficulty of designating an input point and the problem of a poor input sense of a user.
The present invention has been made in consideration of the above problems and aims at providing a coordinate input apparatus and its control method capable of providing a stable and good input sense, irrespective of different input conditions such as a vibration input position, a pen pressure and a pen angle.
Further, with a conventional coordinate input apparatus, however, the amplitude of vibration detected with a vibration sensor changes with a vibration propagation path length from the vibration applying point of a vibration input pen to the vibration sensor on a vibration propagation plate, a load (writing pressure) of the vibration input pen, and an incident angle of vibration.
Therefore, the vibration level detected with the sensor on the vibration propagation plate changes and a stable detection may not be obtained. In order to solve this problem, it has been proposed for the calculation of a coordinate value to measure the detection levels of all sensors and set the amplification factor of an amplifier of each sensor in accordance with the detected level, or to set a drive voltage of a vibration input pen specific to each sensor and drive the vibration input pen at the set drive voltage. With this structure, however, the vibration input pen is required to be driven as many as twice the number of vibration sensors so that it is disadvantageous from the viewpoint of a consumption power and sampling rate.
The invention has been made in consideration of the above problem and aims at providing a coordinate input apparatus and its control method capable of detecting vibration at a stable signal level and detecting a coordinate value at high precision.
It is another object of the present invention to provide a coordinate input apparatus and its control method capable of setting a drive state of the apparatus in accordance with a designated point on an input plane and a distance of each sensor and capable of detecting vibration at a stable signal level at any position on the input plane.
It is a further object of the present invention to provide a coordinate input apparatus and its control method capable of reducing the number of driver circuits and the operation amount thereof and preventing an increase in a consumption power and a reduction of a sampling rate, by calculating a distance between a designated point on an input plane and each sensor and setting the drive state of the system.
Still further, with a conventional coordinate input apparatus, however, if there is any obstacle between a vibration input pen and a sensor, the detection level of the sensor lowers so that some problem occurs in a waveform detection process.
For example, in an ordinary input state of a user, the user sometimes designates an input point with the hand in touch with a vibration propagation plate. In this case, if the hand is on a path between a vibration input pen and a sensor, i.e., on a path along which vibration generated by the vibration input pen propagates toward the sensor, then the vibration amplitude attenuates and the detection level of the sensor lowers. Such a phenomenon appears not only when a hand is in touch with the vibration propagation plate but also when a substance is placed on the plate or stains are attached to the surface of the plate.
The signal processing circuit has been designed therefore by considering the attenuation of the detection level, so that a large power source voltage becomes necessary for retaining a proper dynamic range.
The present invention has been made in consideration of the above problem and aims at providing a coordinate input apparatus capable of stably detecting an input position and retaining a detection level even in a low voltage operation.